1. Field
The present disclosure relates to an apparatus and a method for preparing a manganese oxide-titania catalyst. More particularly, the disclosure relates to an apparatus and a method for preparing a manganese oxide-titania catalyst by chemical vapor condensation (CVC), which allows mass production of the catalyst with reduced process stages and provides high decomposition efficiency of organic compounds.
2. Description of the Related Art
Volatile organic compounds (VOCs) have high vapor pressures and thus are easily evaporated into the atmosphere. In the atmosphere, they photochemically react with nitrogen oxides (NOx) under sunlight to produce photochemical oxides such as ozone (O3), which act as secondary pollutants. Furthermore, VOCs are harmful to the human health since a lot of carcinogenic chemicals are included and are the cause of ozone depletion, global warming, photochemical smog, offensive odor, or the like.
Benzene, a typical example of the VOCs, has serious effects on human health, including inactivation of the central nervous system and increased carcinogenic risks upon exposure for a long time. When test animals were exposed to benzene at high concentration for a long period of time, cancer was induced. It is known that a long-term exposure even at low concentration increases carcinogenic risks in human. Another typical example of the VOCs, toluene, is also known to negatively affect the central nervous system.
According to the data from the Ministry of Environment in 2000, about 700,000 tons of VOCs are emitted annually in Korea. Among them, the emission from the coating industry is the largest with about 52% of the total, the remaining mainly the automobiles, gas stations, printing factories, paint manufacturing, laundries, and so forth.
In general, VOCs are processed by direct combustion, adsorption, catalyzed oxidation, or the like.
Established in 1950s, the direct combustion method is a technique of burning VOCs using heat resulting from combustion of a fuel. Although the processing efficiency is high, the method is disadvantageous in that large amounts of nitrogen oxides (NOx) are produced during the combustion at high temperature and the fuel costs a lot.
The adsorption method is a technique of physically and/or chemically adsorbing VOCs onto activated carbon and is frequently used for medium-to-low concentration emissions. However, since the activated carbon loses its processing efficiency after prolonged use, it has to be replaced frequently. As a result, installation and maintenance costs are high.
The catalyzed oxidation method is a technique of oxidizing VOCs using a catalyst prepared by dispersing precious metal such as platinum (Pt) or palladium (Pd) on a support such as alumina, cordierite or mullite. Although the catalyst life is fairly long, the precious metal such as Pt and Pd is expensive and the catalytic activity is lowered at high temperatures of 500° C. or above.
Recently, the manganese oxide catalyst having good oxidation reactivity is gaining attentions and technologies are being developed with regard thereto. Manganese oxide is widely known as a catalyst that decomposes ozone.
Since the oxygen radical (O*) produced during the decomposition of ozone has excellent oxidation reactivity, the catalyst is utilized in various oxidation reactions. Among the manganese oxide catalysts, manganese dioxide (MnO2) is known to have the best ozone decomposing ability.
For example, U.S. Pat. No. 4,871,709 (Prior Patent Document 1) describes that manganese oxide has been well known as an ozone cracking catalyst and proposes a method for preparing the manganese oxide catalyst. And, Japanese Patent Publication No. S51-71299 (Prior Patent Document 2) discloses a method of preparing activated manganese dioxide by adding potassium permanganate to an acidic aqueous solution of manganese salt and aging the solution.
In addition to the function as the ozone cracking catalyst, manganese oxide is also known to have superior ability of decomposing VOCs. Usually, manganese oxide is supported on a support such as ceramic to be used as a catalyst for decomposing ozone or organic compounds. For example, Prior Patent Document 1 describes dipping a ceramic fiber aggregate in a solution of manganese nitrate, contacting the dipped aggregate with an ammonia gas stream to convert Mn(NO3)2 into Mn(OH)2, and then drying and calcining the resultant aggregate in the air.
As described, ceramic is mainly used as the support of manganese oxide. Recently, titania (TiO2) is preferred among the ceramic materials. With good interactivity with manganese oxide and superior adsorption ability, titania is known to improve decomposition efficiency of organic compounds. For example, Korean Patent No. 0589203 (Prior Patent Document 3) discloses manganese oxide-titania aerogel catalysts, a method for preparing the same and oxidative destruction of chlorinated aromatic compounds using the same.
At present, manganese oxide-titania catalysts are usually prepared by wet methods such as coprecipitation or impregnation. However, since the wet method requires a number of steps, including dissolution, evaporation, drying, pulverization and sintering, a long time is required for the preparation of the catalyst and application o mass production is difficult. Furthermore, the wet method does not provide high decomposition efficiency of organic compounds.
A method for preparing the manganese oxide-titania catalyst by aerogel synthesis has been proposed. That is, Prior Patent Document 3 discloses an aerogel synthesis method of preparing a gel by adding an acid catalyst to a solution of manganese oxide and an inorganic gel source as a precursor to titanium oxide, followed aging, solvent exchange, supercritical drying and heat treatment. However, the aerogel synthesis method is not practicable because it requires the gel aging process of days or more and the expensive supercritical drying process.
In addition, a method of preparing a nanostructured catalyst via a flame synthesis process instead of the liquid-phase process may be considered. However, the flame synthesis process is restricted in that the precursor supply speed is limited. As a result, catalyst production yield is low and mass production in commercial scale is difficult.    [Prior Patent Document 1] U.S. Pat. No. 4,871,709    [Prior Patent Document 2] Japanese Patent Publication No. S51-71299    [Prior Patent Document 3] Korean Patent No. 0589203